Embodiments of the present invention generally relate to implantable medical devices, and more particularly to implantable and external medical devices that utilize segment variations across multiple heart rate ranges to trend coronary burden.
An implantable medical device is implanted in a patient to monitor, among other things, electrical activity of a heart and to deliver appropriate electrical and/or drug therapy, as required. Implantable medical devices (“IMDs”) include for example, pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (“ICD”), and the like. The electrical therapy produced by an IMD may include, for example, pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g., tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g., cardiac pacing) to return the heart to its normal sinus rhythm. Ischemia and AMI represent related types of acute coronary burden.
Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to heart tissue. Ischemia arises during angina, acute myocardial infarction, coronary angioplasty, and any other condition that compromises blood flow to a region of tissue. Ischemia can occur as a result of increased myocardial oxygen demand, reduced myocardial oxygen supply, or both. In the presence of a coronary obstruction (e.g., blockage of an artery), an increase of myocardial oxygen requirements caused by exercise, tachycardia, and the like leads to a transitory imbalance termed “demand ischemia”. Demand ischemia develops in patients with increased demand for cardiac output or perfusion when the myocardial blood supply may be marginal or inadequate. Demand ischemia may be responsible for many episodes of chronic stable angina and usually manifests as a predictable angina that occurs during increased activity. Demand ischemia can be evaluated by means of an exercise stress test and is depicted as a transient depression in the ST segment of the cardiac signal associated with exertion during exercise.
In other situations, an imbalance between the need for myocardial oxygen and the ability to provide myocardial oxygen maybe caused by an acute reduction of oxygen supply (e.g., coronary vasospasm). In addition, the imbalance may be due to a reduction or cessation of coronary blood flow (e.g., within arteries and veins) as a result of platelet aggregates or thrombi, which is termed “supply ischemia”. Supply ischemia manifests as angina at rest or during exercise (e.g., Prinzmetal type angina) and is often responsible for myocardial infarction (“MI”) and most episodes of unstable angina (“UA”). Unstable angina may result from spontaneous thrombus formation and is not necessarily associated with exercise or stress. The onset of UA is therefore unpredictable and patients having UA should seek medical attention.
Implantable medical devices are utilized today for monitoring cardiac signals and delivering certain therapies based arrhythmias detected from the cardiac signals. In general, IMDs have limited memory space for storing cardiac signals and/or characteristics associated with the cardiac signals. For example, the IMD may store cardiac signals for a limited number of cardiac cycles, such as surrounding an event of interest. The stored cardiac signals are later telemetered from the IMD for analysis.
However, conventional IMDs have not been able to detect or record sufficient information to enable a later analysis and diagnosis of supply or demand type ischemia. For example, IMDs have been proposed with the capability to store information regarding variations in ST segments for a limited number of cardiac cycles. However, conventional IMDs do not store a sufficient amount, nor type, of ST segment related information to facilitate analysis and diagnosis of supply and demand ischemia.
A need remains for an IMD capable of storing information that enables the ability to evaluate if ischemia is present, categorize the type of ischemia as demand ischemia or supply ischemia, and determine the relative frequency in which ischemia occurs over a specified period of time.